FIG. 11 is a perspective view showing a conventional compact wheel type tractor 97. This tractor 97 is a rear drive four-wheel vehicle, in general, this kind of wheel type tractor is commonly attached with a farm working machinery such as a tilling attachment, a levee forming attachment, a soil disinfecting attachment, a cultivation bed making attachment, a seed tuber planting attachment, and a mulch working attachment which are moved upward-and-downward and driven by hydraulic pressure and gears or the like. Therefore, since the wheel type tractor 97 attached with a farm working machinery is likely to lose easily a front-and-rear balance of vehicle and ground contact pressure of tires is high, there is the case that running performance on soft ground or irregular ground is not sufficient, especially in the case of a compact tractor, traction force is sometimes insufficient due to the compactness. In such a case, if the wheel type tractor 97 can be improved to a compact crawler type tractor, running performance on soft ground or irregular ground can be improved due to the reduction of ground contact pressure, and at the same time, traction force can be increased.
As such technology, for example, one improving to a new compact crawler type tractor by sharing a steering mechanism part of a wheel type tractor is exemplified (Patent Document 1). The Patent Document was created in view of the point that turning characteristics is not good due to the difference of a turning mechanism between a crawler type and a wheel type, more specifically, in the case of the conversion of a wheel type tractor into a crawler type one, turning characteristics is not good to turn a vehicle with decelerating a crawler at the inner side of turning by braking a driving sprocket on one side while operating a braking device on one side by depressing either a right or a left break pedal at the inner side of turning, and intended to enable to turn vehicle of a compact crawler type tractor by rotating operation of a circular steering handle and also to reduce cost by sharing the parts with utilizing a steering mechanism part provided on a wheel type tractor.
In addition, while a wheel type working vehicle can drive if wheels contact with ground even in the case that a balance is rather poor, there is a case that traction force is not sufficient because a front end lifts up in a crawler type working vehicle. In addition, since a compact crawler type tractor is mainly used in ill-drained paddy fields, it is necessary to set ground clearance high. Thus, a technology by which a front-and-rear balance and ground clearance of a tractor are adjusted with a simple and inexpensive structure, has been already disclosed (Patent Document 2). In Patent Document 2, a compact crawler type tractor was disclosed, which is equipped with the right and left crawler type running units that a crawler is windingly mounted between a driving sprocket wheel and a driven idler, and the compact crawler type tractor is made so as to enable to adjust a installing position of a crawler frame supporting the driving sprocket wheel and the driven idler in the front-and-rear direction and/or in the up and down direction to a vehicle frame. The compact drawler type tractor disclosed in Patent Document 2 is estimated that the tractor part is targeting at a middle-size or a large-size, and a front edge of a supporting member which attaches a crawler frame supporting a driving sprocket wheel and a driven idler to a vehicle, is mounted at the lower part of the tractor in front of a front edge of the crawler type running unit.
On the other hand, especially in a middle-sized or large-sized tractor, a technology by which a driving system of a compact crawler type tractor is simplified and turning characteristics is improved, was developed (Patent Document 3). For example, Patent Document 3 disclosed a compact crawler type tractor of a front drive crawler type tractor in which a hydrostatic transmission turning mechanism (hereinafter referred to as “HST mechanism for turning”) to turn a vehicle is connected with a driving system located behind a backward-and-forward movement changeover mechanism for carrying a vehicle backward or forward, wherein the HST mechanism for turning is divided into a variable displacement pump and a fixed displacement motor, and the fixed displacement motor is connected with an input shaft of a planetary gear type differential mechanism without making a gear speed change mechanism for auxiliary speed change and the HST pump for turning work together. This tractor is connected with the above variable displacement pump by an output after auxiliary speed change of a running speed change mechanism, it is said that it is possible to keep a turning radius constant and to ensure the same operation feeling as a wheel tractor by making a vehicle speed proportionate with rotation of a steering motor.
In addition, as a compact crawler type tractor being light-weight, simple and low cost, and being provided with a construction of farm working machinery having a good front-and-rear balance and good operability in spite of short overall length, a front drive compact crawler type tractor was also disclosed, wherein a mission case and an axle case are constructed in an integrated fashion, and the mission case is supported by a front-and-rear connection frame arranged horizontally between a pair of right and left crawler running units, and the axle case is located in front of the connection frame (Patent Document 4).    Patent Document 1: JP-A-2000-159143    Patent Document 2: JP-A-2002-145134    Patent Document 3: JP-A-2004-17841    Patent Document 4: JP-A-2004-66913